Plane illumination apparatus and backlight apparatus

ABSTRACT

A plane illumination apparatus has an optical device, an irradiation unit, and a light guide plate. The irradiation unit makes the coherent light beams scan the surface of the optical device, the light guide plate comprises a light take-out portion to take out coherent light beams to outside while making coherent light beams propagate between a first end face on which coherent light beams from the optical device are incident and a second end face provided to face the first end face, the specific zone is provided inside the light take-out portion or along the first end face, or along the second end face, the light take-out surface is a third end face that is connected to the first and second end faces, and the irradiation unit is provided at a rear side of a fourth end face that is an opposite side to the light take-out surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plane illumination apparatus and abacklight apparatus that use a light source for emitting coherent lightbeams.

2. Description of Related Art

As a backlight apparatus used in liquid crystal panels or the like, asystem in which light is incident on an edge of a light guide plate,repeatedly reflected between two opposing surfaces by total reflection,and then taken out by a diffusion device or the like is known. This typeof backlight apparatus includes backlight apparatuses using a coldcathode fluorescent lamp as a light source and also, recently, backlightapparatuses using an LED as a light source.

When a cold cathode fluorescent lamp is used, there is a problem in thatit is difficult to make a backlight apparatus thin and power consumptionincreases. When an LED is used, although it is possible to make abacklight apparatus thin, since an LED is a uniform diffusionillumination device, it is difficult to make every light incident on athin light guide plate with no leakage, hence loss is caused.

In contrast to above, a laser beam is excellent in straightness, andhence considered to improve light incidence efficiency.

However, when a laser beam is used as a light source, speckles due tohigh coherency of laser are generated. Speckles are a spotted patternwhich is formed when a coherent light beam such as a laser beam isemitted to a scattering plane. If speckles are generated on a screen,they are observed as spotted luminance unevenness, i.e. brightnessunevenness, thus becoming a factor of having physiologically adverseaffect on an observer. It is considered that the reason why speckles aregenerated in the case of using coherent light beams is that coherentlight beams reflected from respective portions of a scattering andreflecting plane such as a screen have very high coherency so thatcoherent light beams interfere with one another to generate speckles.For example, a theoretical review of the generation of speckles is madein detail in Speckle Phenomena in Optics, Joseph W. Goodman, Roberts &Co., 2006.

As discussed above, in the system using a coherent light source, sincethere is a problem of generation of speckles unique to the coherentlight source, techniques for suppressing the generation of speckles havebeen proposed. For example, Japanese Patent Laid-Open No. 6-208089discloses a technique in which a laser beam is emitted to a scatteringplate, scattered light beams obtained therefrom are guided to a spatiallight modulator, and the scattering plate is driven to rotate by amotor, thus reducing speckles.

SUMMARY OF THE INVENTION

Not only backlight apparatuses, speckles are problems in a variety ofapparatuses having an illumination apparatus for illuminating anillumination zone with coherent light beams. Coherent light beams, forexample laser beams as a typical example, show excellent straightnessand can emit a light of extremely high energy density. Therefore, it ispreferable for illumination apparatuses actually developed to design theoptical path of coherent light beams in accordance with thecharacteristics of coherent light beams.

The inventors have extensively researched under consideration of thepoints discussed above, and as a result, the inventors have contrivedthe invention regarding a plane illumination apparatus and a backlightapparatus which illuminate a specific zone repeatedly with coherentlight beams that are then diffused and taken out to the outside withspeckles made inconspicuous. Moreover, the inventors have proceeded withresearches and succeeded in improvement in the illumination apparatus toconstantly prevent the generation of a region extremely bright in aspecific zone illuminated with coherent light beams. Namely, the purposeof the present invention is to provide a plane illumination apparatusand a backlight apparatus that are capable of making specklesinconspicuous and effectively suppressing the generation of brightnessunevenness in a specific zone.

In order to solve the problems above, according to an aspect of thepresent invention, there is provided a plane illumination apparatuscomprising:

an optical device configured to be capable of diffusing coherent lightbeams from respective points to an entire region of the correspondingareas in a specific zone;

an irradiation unit configured to irradiate the coherent light beams tothe optical device so that the coherent light beams scan a surface ofthe optical device; and

a light guide plate configured to make coherent light beams that arereflected at a surface of the optical device or that have passed throughthe optical device propagate and to comprise a light take-out surfacefrom which the coherent light beams are taken out to outside,

wherein the irradiation unit makes the coherent light beams scan thesurface of the optical device by changing propagation directions of thecoherent light beams,

the light guide plate comprises a light take-out portion configured totake out coherent light beams to outside while making coherent lightbeams propagate between a first end face on which coherent light beamsfrom the optical device are incident and a second end face that isprovided to face the first end face,

the specific zone is provided inside the light take-out portion or alongthe first end face, or along the second end face,

the light take-out surface is a third end face that is connected to thefirst and second end faces, and

the irradiation unit is provided at a rear side of a fourth end facethat is an opposite side to the light take-out surface of the lighttake-out portion.

According to the present invention, it is possible to provide a planeillumination apparatus and a backlight apparatus that are capable ofmaking speckles inconspicuous and effectively suppressing the generationof brightness unevenness in a specific zone.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a) and 1(b) are views showing a schematic configuration of aplane illumination apparatus according to an embodiment of the presentinvention;

FIGS. 2(a) and 2(b) are views showing a schematic configuration of aplane illumination apparatus according to a modification of FIGS. 1(a)and 1(b);

FIG. 3 is a view explaining the operational principle of an illuminationapparatus 40 of FIGS. 1(a) and 1(b);

FIG. 4 is a view explaining a state in which an image of a scatteringplate is formed on a hologram recording medium 55 as interferencefringes;

FIG. 5 is a view explaining a state in which an image of a scatteringplate is reproduced using interference fringes generated in the hologramrecording medium 55 obtained through an exposure process of FIG. 4;

FIG. 6 is a view explaining a scanning route of a scanning device 65;

FIG. 7 is a view showing results of measuring speckle contrasts in thecases where the hologram recording medium 55 was used and not used;

FIG. 8 is a view showing an example of an irradiation unit provided witha scanning device rotatable in two axes directions; and

FIG. 9 is a view showing an example of making a parallel beam incidenton the hologram recording medium 55.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be explained withreference to the drawings. In the accompanying drawings of the presentdescription, in order for simplifying drawings and easy understanding,the scale, the ratio of height to width, etc., are appropriatelymodified or enlarged.

A plane illumination apparatus according to an embodiment of the presentinvention is applicable to a backlight apparatus to be installed into,for example, a liquid crystal panel or the like. However, notnecessarily be limited to an application to a backlight apparatus, aplane illumination apparatus according to an embodiment of the presentinvention can be used as a plane illumination apparatus for illuminationat a specific size of plane.

FIGS. 1(a) and 1(b) are views showing a schematic configuration of aplane illumination apparatus according to an embodiment of the presentinvention. FIG. 1(a) is a plan view and FIG. 1(b) is a sectional view ofFIG. 1(a). The plane illumination apparatus shown in FIGS. 1(a) and 1(b)is provided with an optical device 50, an irradiation unit 60, and alight guide plate 30. In the present description, an apparatus providedwith the optical device 50 and the irradiation unit 60 is referred to asan illumination apparatus 40 and an apparatus provided with theillumination apparatus 40 and the light guide plate 30 is referred to asa plane illumination apparatus.

The irradiation unit 60 irradiates the optical device 50 with coherentlight beams so that the coherent light beams scan the surface of theoptical device 50. The irradiation unit 60 has a laser source 61 thatemits coherent light beams, a scanning device 65 that scans the surfaceof the optical device 50 with the coherent light beams emitted from thelaser source 61, a Fresnel lens (a divergence-angle controlling part) 68that restricts the divergence angle of coherent light beams reflected bythe scanning device 65, and a mirror member 69 that reflects coherentlight beams that have passed through the Fresnel lens 68 to guide thecoherent light beams to the optical device 50.

The optical device 50 has a hologram recording medium 55 that canreproduce an image of a scattering plate in an illumination zone (aspecific zone) LZ. The details of the hologram recording medium 55 willbe explained later. A plurality of recording areas r1 to rn are providedon the hologram recording medium 55. Coherent light beams reflected bythe scanning device 65 within different reflection angle ranges areincident on the plurality of recording areas r1 to rn. The coherentlight beams scan the corresponding recording areas. An interferencefringe is formed on each of the recording areas r1 to rn. When acoherent light beam is incident on each recording area, a coherent lightbeam diffracted by the interference fringe is emitted as diverging light(diffused light).

As described above, on each of the recording areas r1 to rn on thehologram recording medium 55, a coherent light beam from the scanningdevice 65 within the corresponding reflection angle range is incidentand scans the recording area.

The recording areas r1 to rn on the hologram recording medium 55 arearranged in tight contact with one end face of the light guide plate 30.At least part of the light guide plate 30 is provided with a lighttake-out portion 31. The light take-out portion 31 is provided with theillumination zone LZ to be illuminated with coherent light beams fromthe optical device 50.

Coherent light beams incident on respective points in the recordingareas r1 to rn on the hologram recording medium 55 become diffused lightand create line images LZ1 to LZn in the corresponding areas in theillumination zone LZ. For example, if there are an n number (n being aninteger of 2 or more) of recording areas r1 to rn, line images LZ1 toLZn are created in the n number of corresponding areas in theillumination zone LZ.

The illumination zone LZ is provided inside the light take-out portion31, along a first end face 31 a closest to the optical device 50 or asecond end face 31 b farthest from the optical device 50.

As shown in FIG. 1(b), the light take-out portion 31 is provided withthe first end face 31 a on which coherent light beams from the opticaldevice 50 are incident, the second end face 31 b provided to face thefirst end face 31 a, and third and fourth end faces 31 c and 31 dconnected to the first and second end faces 31 a and 31 b. The lighttake-out portion 31 makes coherent light beams incident on the first endface 31 a propagate towards the second end face 31 b while reflectingthe coherent light beams at the third and fourth end faces 31 c and 31d, and takes out the coherent light beams little by little to theoutside from the third end face 31 c during propagation. In this way,the plane illumination apparatus functions to radiate light of uniformbrightness from the enter region of the third end face 31 c.

Diffused light beams from the recording areas r1 to rn on the hologramrecording medium 55 are reflected at the opposing two surfaces of thelight guide plate 30 by total reflection and reach the first end face 31a of the light take-out portion 31. Accordingly, diffused light beamsfrom the hologram recording medium 55 are incident on the first end face31 a with almost no leakage.

The laser source 61, the scanning device 65, and the Fresnel lens 68 inthe irradiation unit 60 are arranged at the rear surface side of thefourth end face 31 d, that is, the rear side of the face 31 d on theopposite side to the irradiation surface (the third end face 31 c) ofthe light take-out portion 31. By providing the Fresnel lens 68, theratio of coherent light beams that are not incident on the mirror member69 from the scanning device 65 can be reduced, hence the utilizationefficiency of coherent light beams can be improved. A thin thickness isa feature of the Fresnel lens 68. The reason why the Fresnel lens 68 isused is that, in the present embodiment, the Fresnel lens 68 has to beprovided at the rear side of the fourth end face 31 c, and hence thereis a limitation on space in the depth direction. If the limitation onthe length in depth of the plane illumination apparatus is not strict, athick optical member other than the Fresnel lens 68 may be used. Or adiffraction device such as a hologram recording medium recorded with alens diffraction condition may be used instead of the Fresnel lens 68.

If the utilization efficiency of coherent light beams is not viewed as aproblem, the Fresnel lens 68 may be omitted. Accordingly, the Fresnellens 68 itself is not always be a necessary component.

Coherent light beams that have passed through the Fresnel lens 68 areincident on the mirror member 69. The mirror member 69 reflects thecoherent light beams that have passed through the Fresnel lens 68 in thedirection of the hologram recording medium 55. In more specifically, themirror member 69 is provided at the rear side of the opposite surface tothe reflection surface of the plane illumination apparatus, to reflectcoherent light beams from the rear side to the irradiation plane.

As described above, coherent light beams are incident on the hologramrecording medium 55 from the rear surface side of the plane illuminationapparatus. Therefore, the hologram recording medium 55 is not requiredto be provided obliquely, so that the gap between the hologram recordingmedium 55 and the light take-out portion 31 can be made narrower,thereby a plane illumination apparatus having a narrow frame can berealized.

The mirror member 69 is not always be a necessary component. The mirrormember 69 may be omitted, for example, if the optical device 50 can beprovided in an oblique direction to the propagation direction ofcoherent light beams that have passed through the Fresnel lens 68 sothat an illumination zone can be illuminated with diffused light beamsreflected by the optical device 50.

The light guide plate 30 including the light take-out portion 31 thereinis configured by sandwiching an acrylic plate with a scattering sheetand a reflection sheet. On the reflection sheet, reflection dots areprinted with a white ink. The scattering sheet corresponding to thethird end face 31 c is a light take-out surface. The reflection sheetcorresponding to the fourth end face 31 d is a reflection surface. Byadjusting the density of the reflection dots on the reflection sheet,light of uniform brightness can be taken out from the scattering sheetside.

FIGS. 1(a) and 1(b) show an example in which the hologram recordingmedium 55 of the optical device 50 is provided in tight contact with thelight guide plate 30. However, the hologram recording medium 55 and thelight guide plate 30 are arranged apart from each other. FIGS. 2(a) and2(b) are a modification of FIGS. 1(a) and 1(b), showing a planeillumination apparatus having a hologram recording medium 55 and a lightguide plate 30 arranged apart from each other. FIG. 2(a) is a plan viewand FIG. 2(b) is a sectional view.

The light guide plate 30 of FIGS. 2(a) and 2(b) is provided with a lighttake-out portion 31 at the almost entire region. The light guide plate30 has three end faces (second, fifth and sixth end faces 31 b, 31 e and31 f) as mirror surfaces, except for a first end face 31 a that is anincidence surface for diffused light from an optical device 50, a lighttake-out surface (a third end face 31 c), and a fourth end face 31 dthat faces the light take-out surface.

Diffused light from each of recording areas r1 to rn of the hologramrecording medium 55 is directly incident on the light guide plate 30 atthe first end face 31 a side without being reflected anywhere andpropagates towards the second end face 31 b side while being reflectedat the third and fourth end faces 31 c and 31 d.

While propagating, if part of light reaches the second, fifth and sixthend faces 31 b, 31 e and 31 f, it is reflected by total reflectionbecause these end faces are mirror surfaces. Therefore, it is possibleto take out light efficiently from the light take-out surface (the thirdend face 31 c). It is not always necessary that all of the second, fifthand sixth end faces 31 b, 31 e and 31 f are mirror surfaces. Any ofthese end faces may be a mirror surface.

It is preferable for the plane illumination apparatus of FIGS. 2(a) and2(b) that, since the hologram recording medium 55 and the light guideplate 30 are apart from each other, a structural improvement is made atthe incidence surface side of the light guide plate 30 in order thatdiffused light from the hologram recording medium 55 is easily incidenton the light guide plate 30. For example, as an example, the light guideplate 30 may be configured to be thick at the diffused-light incidencesurface side so that diffused light is easily incident thereon.

Also in the plane illumination apparatus of FIGS. 2(a) and 2(b), likethe plane illumination apparatus of FIGS. 1(a) and 1(b), an illuminationzone LZ is provided inside the light take-out portion 31, along thefirst end face 31 a closest to the optical device 50 or the second endface 31 b farthest from the optical device 50.

Also in the plane illumination apparatus of FIGS. 1(a) and 1(b), likethe plane illumination apparatus of FIGS. 2(a) and 2(b), at least anyone of the second, fifth and sixth end faces 31 b, 31 e and 31 f may bea mirror surface.

FIG. 3 is a view explaining the operational principle of theillumination apparatus 40. In FIG. 3, for easy explanation, somecomponents in the illumination apparatus 40 are only shown. Hereinafter,the basic operational principle of the illumination apparatus 40 will beexplained using FIG. 3.

The hologram recording medium 55 of the optical device 50 can receivecoherent light beams emitted from the irradiation unit 60 asreproduction illumination light beams La and diffract the coherent lightbeams at high efficiency. Above all, the hologram recording medium 55 isconfigured to be capable of reproducing an image 5 of a scattering plate6 on the illumination zone LZ by diffracting coherent light beamsincident on its respective positions, in other words, respective microzones which should be called respective points.

The irradiation unit 60 is configured so that the optical device 50 usescoherent light beams emitted to the hologram recording medium 55 to scanthe hologram recording medium 55. Therefore, at a moment, theirradiation unit 60 irradiates a micro zone on the surface of thehologram recording medium 55 with coherent light beams.

Coherent light beams emitted from the irradiation unit 60 to scan thehologram recording medium 55 are incident on respective positions, i.e.respective micro zones on the hologram recording medium 55 at incidentangles that satisfy diffraction conditions of the hologram recordingmedium 55. Coherent light beams incident on respective positions of thehologram recording medium 55 from the irradiation unit 60 are diffractedby the hologram recording medium 55 to illuminate the specific zonesthat are overlapped with one another at least partially. Above all inthe embodiment described here, coherent light beams incident onrespective positions of the hologram recording medium 55 from theirradiation unit 60 are diffracted by the hologram recording medium 55to illuminate the same illumination zone LZ. In more detail, as shown inFIG. 3, a coherent light beam incident on any position in each of therecording areas r1 to rn of the hologram recording medium 55 from theirradiation unit 60 reproduces an image 5 of a scattering plate 6 in amanner that the image is superimposed on the corresponding area in theillumination zone LZ. Namely, coherent light beams incident on anyrespective positions in the recording areas r1 to rn of the hologramrecording medium 55 from the irradiation unit 60 are diffused, i.e.spread by the optical device 50 to be incident on the correspondingareas of the illumination zone LZ to create line images LZ1 to LZn,respectively.

As for the hologram recording medium 55 that enables the diffraction ofcoherent light beams described above, in the example shown, areflection-type volume hologram using photopolymer is used. FIG. 4 is aview explaining a state in which an image of a scattering plate isgenerated on the hologram recording medium 55 as interference fringes.Here, the scattering plate 6 is a reference member for scattering lightand it does not matter what a configuration the scattering plate 6 has.

As shown in FIG. 4, the hologram recording medium 55 is produced usingscattered light beams from an actual scattering plate 6 as object beamsLo. FIG. 4 shows a state in which a hologram photosensitive material 58that shows photosensitivity to become the hologram recording medium 55is exposed by reference beams Lr and object beams Lo, both beingcoherent light beams that show coherence to each other.

As for the reference beams Lr, for example, laser beams from the lasersource 61 that oscillates laser beams in a specific wavelength range areused. The reference beams Lr pass through a condenser element 7 made ofa lens and are incident on the hologram photosensitive material 58. Inthe example shown in FIG. 4, laser beams to become the reference beamsLr are incident on the condenser element 7 as a parallel light flux thatis parallel with the optical axis of the condenser element 7. By passingthrough the condenser element 7, the reference beams Lr are shaped, i.e.converted, from a parallel light flux into a convergent light flux andincident on the hologram photosensitive material 58. On this occasion, afocal point FP of the convergent light flux Lr is located at a positionbeyond the hologram photosensitive material 58. In other words, thehologram photosensitive material 58 is located between the condenserelement 7 and the focal point FP of the convergent light flux Lrcollected by the condenser element 7.

Next, the object beams Lo are incident on the hologram photosensitivematerial 58 as scattered light from the scattering plate 6 made of opalglass, for example. In the example shown in FIG. 4, the hologramrecording medium 55 to be produced is a reflection type and the objectbeams Lo are incident on the hologram photosensitive material 58 on theopposite side to the reference beams Lr. It is a precondition that theobject beams Lo are coherent with the reference beams Lr. Therefore, forexample, it is possible to separate laser beams oscillated by the samelaser source 61 and use one of the separated ones as the reference beamsLr and the other as the object beams Lo described above.

In the example shown in FIG. 4, a parallel light flux that is parallelwith the direction of normal to the plate surface of the scatteringplate 6 is incident on the scattering plate 6 and scattered, and thenthe scatted beams that have passed through the scattering plate 6 areincident on the hologram photosensitive material 58 as the object beamsLo. According to this method, when an isotropic scattering plateavailable at usually low cost is used as the scattering plate 6, theobject beams Lo from the scattering plat 6 can be easily incident on thehologram photosensitive material 58 at roughly constant intensitydistribution. Moreover, according to this method, although depending onthe degree of scattering by the scattering plate 6, the object beams Locan be easily incident on respective positions of the hologramphotosensitive material 58 at roughly constant intensity from the entireregion of a light-emitting surface 6 a of the scattering plate 6. Insuch a case, it is achievable that light beams incident on respectivepositions of the obtained hologram recording medium 55 reproduce images5 of the scattering plate 6 at similar brightness and reproduced images5 of the scattering plate 6 are observed at roughly constant brightness.

As described above, when the hologram photosensitive material 58 isexposed by the reference beams Lr and object beams Lo, interferencefringes caused by the interference between the reference beams Lr andobject beams Lo are generated and interference fringes of these lightbeams are recorded in the hologram photosensitive material 58 as someform of pattern, i.e. an refractive index modulation pattern, as oneexample in a volume hologram. Thereafter, an appropriate post-treatmentcorresponding to the type of the hologram photosensitive material 58 isapplied, thereby obtaining the hologram recording medium 55.

The hologram recording medium 55 in the present embodiment has aplurality of recording areas r1 to rn, so that an interference fringe isformed in each recording area by a technique shown in FIG. 4. In thepresent embodiment, the hologram recording medium 55 is used forillumination. Therefore, when an interference fringe is formed bycollecting reference beams, it is required to change the interferencefringe per recording area. However, when an interference fringe isformed by using reference beams converted into parallel beams, theinterference fringe to be formed per recording area may be the identicalone and it is not required to change the type of interference fringe perrecording area.

FIG. 5 is a view explaining a state in which an image of a scatteringplate is reproduced using interference fringes formed in the hologramrecording medium 55 obtained through an exposure process of FIG. 4. Asshown in FIG. 5, the hologram recording medium 55 produced with thehologram photosensitive material 58 of FIG. 4 meets its Bragg conditionby means of light beams that have the same wavelength as the laser beamsused in the exposure process and propagate in a reverse direction of thereference beams Lr along an optical path of the reference beams Lr.Namely, as shown in FIG. 5, a diverging light flux that diverges from areference point SP located with respect to the hologram recording medium55 so as to have the same positional relationship as the relativeposition of the focal point FP in FIG. 4 with respect to the hologramphotosensitive material 58 in the exposure process and that has the samewavelength as the reference beams Lr in the exposure process isdiffracted by the hologram recording medium 55 as the reproductionillumination light beams La, thereby creating the reproduced image 5 ofthe scattering plate 6 at a specific location with respect to thehologram recording medium 50 so as to have the same positionalrelationship as the relative position of the scattering plate 6 in FIG.4 with respect to the hologram photosensitive material 58 in theexposure process.

In this occasion, reproduction beams Lb. i.e. beams obtained bydiffracting the reproduction illumination light beams La by the hologramrecording medium 55, for creating the reproduced image 5 of thescattering plate 6 reproduce respective points of the image 5 of thescattering plate 6 as beams propagating in the reverse direction of theoptical path of the object beams Lo propagated towards the hologramphotosensitive material 58 from the scattering plate 6 in the exposureprocess. Moreover, as shown in FIG. 4, object beams Lo emitted fromrespective points of the light-emitting surface 6 a of the scatteringplate 6 in the exposure process are diffused, i.e. spread, to beincident on roughly entire region of the hologram photosensitivematerial 58. Namely, on respective points of the hologram photosensitivematerial 58, the object beams Lo from the entire region of thelight-emitting surface 6 a of the scattering plate 6 are incident. As aresult, information of the entire light-emitting surface 6 a is recordedat respective points of the hologram recording medium 55. It istherefore possible that beams that form a diverging light flux from thereference point SP and function as the reproduction illumination lightbeams La are incident on respective points of the hologram recordingmedium 55 to reproduce the images 5 of the scattering plate 5 having thesame contour as one another at the same location, i.e. the illuminationzone LZ, respectively.

The light beams incident on the hologram recording medium 55 arediffracted in the direction of the illumination zone LZ, hence uselessscattered light can be effectively restricted. Therefore, all of thereproduction illumination beams La incident on the hologram recordingmedium 55 can be effectively used for creating the image of thescattering plate 6.

Next, the configuration of the irradiation unit 60 that emits coherentlight beams to the optical device 50 made of the hologram recordingmedium 55 described above will be explained. In the example shown inFIGS. 1 to 3, the irradiation unit 60 is provided with the laser source61 that generates coherent light beams and the scanning device 65 thatchanges the propagation direction of coherent light beams from the lasersource 61.

The laser source 61 emits, for example, visible light. Or a plurality oflaser sources 61 that emit laser beams of different wavelength rangesmay be used. When a plurality of laser sources 61 are used, it isarranged that the same point on the scanning device 65 is irradiatedwith a laser beam from each laser source 61. With this arrangement, thehologram recording medium 55 is illuminated with reproductionillumination light beams having illumination colors of the laser sources61 mixed with one another.

The hologram recording medium 55 is provided with an n number ofrecording areas r1 to rn so as to correspond to an n number of lineimages LZ1 to LZn to be created in the illumination zone LZ,respectively. On each of the recording areas r1 to rn, a coherent lightbeam within the corresponding reflection angle range from the scanningdevice 65 is incident.

The n number of recording areas r1 to rn can be provided on the hologramrecording medium 55 by irradiating each recording area with a referencebeam Lr and an object beam Lo to form an interference fringe on eachrecording area.

The recording areas r1 to rn may not always necessarily be arranged intight contact with one another but may be arranged with a gaptherebetween. If gaps are provided, coherent light beams incident on thegaps are not used for creating line images LZ1 to LZn, however,practically there is no problem. Or interference fringes may be formedso that recording areas next to each other are overlapped with eachother.

The line images LZ1 to LZn may also not always necessarily be arrangedin tight contact with one another but may be arranged with a gaptherebetween. Even if there are some gaps, practically there is noproblem as far as uniform plane illumination is possible because of thecharacteristics of the light guide plate 30. For a similar reason, asfar as uniform plane illumination is possible, line images next to eachother may be superimposed on each other.

The laser source may be a singe-color laser source or a plurality oflaser sources of different colors, for example, red, green and blue.When a plurality of laser sources are used, the laser sources arearranged so that coherent light beams from the laser sources are emittedto a single point on the scanning device 65. With this arrangement,coherent light beams from the laser sources are reflected by thescanning device 65 at reflection angles corresponding to the incidentangles of coherent light beams from the laser sources, incident on thehologram recording medium 55, diffracted by the hologram recordingmedium 55 separately, and superimposed on one another on theillumination zone LZ, thereby having a combined color, for example,white. Or a scanning device 65 may be provided for each laser source.

For example, when illuminating with white, a color as much closer towhite as possible may be reproduced by providing another laser source,for example, a laser source that emits light in yellow, other than red,green and blue. Therefore, there is no particular limitation on the typeof laser source provided in the irradiation unit 60.

The scanning device 65 changes the propagation direction of a coherentlight beam with time to direct the coherent light beam in differentdirections so that the coherent light beam does not propagate in thesame direction. This results in that the coherent light beam, thepropagation direction of which is changed by the scanning device 65,scans the incidence surface of the hologram recording medium 55 of theoptical device 50.

As described above, since the n number of recording areas r1 to rn areformed on the incidence surface of the hologram recording medium 55,coherent light beams are incident on any of the recording areas inaccordance with the reflection angle of the coherent light beams on thescanning device 65.

In the example shown in FIG. 3, the scanning device 65 includes areflection device 66 having a reflection surface 66 a rotatable aboutone axis line RA1. FIG. 6 is a view explaining a scanning route of thescanning device 65. As understood from FIG. 6, the reflection device 66has a mirror device that has a mirror as the reflection surface 66 arotatable about one axis line RA1. The mirror device 66 is configured tochange the orientation of the mirror 66 a to change the propagationdirection of the coherent light beams from the laser source 61. In thisoccasion, as shown in FIG. 3, the mirror device 66 is provided so as toreceive the coherent light beams from the laser source 61 roughly at thereference point SP.

A coherent light beam, for which final adjustments were made to itspropagation direction by the mirror device 66, can be incident on thehologram recording medium 55 of the optical device 50 as a reproductionillumination light beam La that can become one beam included in adiverging light flux from the reference point SP in FIG. 5. As a result,coherent light beams from the irradiation unit 60 scan the hologramrecording medium 55 and coherent light beams incident on respectivepositions of the hologram recording medium 55 reproduce images 5 of thescattering plate 6 having the same contour on the same location, i.e.the illumination zone LZ.

FIG. 6 is a view for explaining the movement of the reflection device66, having the Fresnel lens 68 and the mirror member 69 omitted. Asshown in FIG. 6, the reflection device 66 is configured to rotate themirror 66 a about one axis line RA1. In the example shown in FIG. 6, therotation axis line RA1 of the mirror 66 a extends in parallel with they-axis of the x-y axis system, that is, the x-y axis system having thex-y plane in parallel with the surface of the hologram recording medium55, defined on the surface of the hologram recording medium 55. Then,the mirror 66 a rotates about the axis line RA1 that is in parallel withthe y-axis of the x-y axis system defined on the surface of the hologramrecording medium 55. Therefore, an incidence point IP of a coherentlight beam from the irradiation unit 60 on the optical device 50 movesin a reciprocating motion in the direction parallel with the x-axis ofthe x-y axis system defined on the surface of the hologram recordingmedium 55. Namely, in the example shown in FIG. 6, the irradiation unit60 emits a coherent light beam to the optical device 50 to scan thehologram recording medium 55 along a straight route.

The scanning device 65 including the mirror device 66 and othercomponents is, as described above, a member rotatable about at least theaxis line A1 and configured with a MEMS, for example. The scanningdevice 65 periodically moves in rotational motion, however, there is noparticular limitation on its rotational frequency as far as it can scanwith coherent light beams at about 1/30 seconds per one cycle for use,for example, in a backlight apparatus with which a human directlyobserves or at higher speed in accordance with the type of image to bedisplayed.

As a practical problem, the hologram photosensitive material 58 mayshrink when the hologram recording medium 55 is produced. In such acase, it is preferable to adjust the recording angles of coherent lightbeams to be entered to the optical device 50 from the irradiation unit60 under consideration of the shrinkage of the hologram photosensitivematerial 58. The wavelengths of coherent light beams generated by thelaser sources 61 do not need to be precisely the same as the wavelengthof the light beam used in the exposure process of FIG. 4 but may beroughly the same.

In a similar reason, even if the propagation direction of a light beamto be incident on the hologram recording medium 55 of the optical device50 does not take precisely the same route as one beam included in adiverging light flux from the reference point SP, an image 5 can bereproduced in the illumination zone LZ. Actually, in the examples shownin FIGS. 3 and 6, the mirror, i.e. reflection plane 66 a of the mirrordevice 66 of the scanning device 65 is inevitably displaced from itsrotational axis line RA1. Therefore, when the mirror 66 a is rotatedabout the rotational axis line RA1 that does not pass through thereference point SP, a light beam to be incident on the hologramrecording medium 55 may not be one of the beams that form a diverginglight flux from the reference point SP. However, practically, an image 5can be substantially reproduced in a manner that the image 5 issuperimposed on the illumination zone LZ by means of coherent lightsfrom the irradiation unit 60 having the shown configuration.

The scanning device 65 may not necessarily be a device for reflectingcoherent light beams but a device for refracting or diffracting coherentlight beams so that coherent light beams san the optical device 50.

Effects of Present Embodiment

Next, the functions of the plane illumination apparatus having theconfiguration described above will be explained. Firstly, theirradiation unit 60 emits coherent light beams to the optical device 50so as to successively scan the n number of recording areas r1 to rn inthe hologram recording medium 55 of the optical device 50. Specifically,the laser source 61 generates coherent light beams having a specificwavelength that propagate along a unidirection. These coherent lightbeams are emitted to the scanning device 65 to change their respectivepropagation directions. More specifically, the scanning device 65reflects each coherent light beam at a reflection angle corresponding toan incident angle from the laser source 61. Reflected coherent lightbeams are incident on the Fresnel lens 68 and refracted so that thediffusion angle does not become large more than necessary. Refractedcoherent light beams are once reflected by the mirror member 69 and thenincident on the hologram recording medium 55 provided diagonally upward.

The Fresnel lens 68 has a diameter corresponding to the width of thefirst end face. The incidence position of a coherent light beam on theFresnel lens 68 changes with time. Thus, the propagation path of acoherent light beam refracted by the Fresnel lens 68 also changes withtime. Therefore, the mirror surface of the mirror member 69 is alsorequired to have a length corresponding to the width of the first endface. Or a plurality of mirror members 69 may be aligned in accordancewith the propagation path of coherent light beams that pass through theFresnel lens 68. In this case, the Fresnel lens 68 may be made of amicrolens array.

The incidence position of a coherent light beam on the hologramrecording medium 55 from the scanning device 65 is shifted with time ineach recording area by the operation of the scanning device 65.

The scanning device 65 makes coherent light beams incident on specificpositions on the hologram recording medium 55 at an incident angle thatmeets the Bragg condition on the respective positions, with thecorresponding specific wavelengths. As a result, the coherent lightbeams incident on the specific positions in the recording area 55reproduce images 5 of the scattering plate 6 in a manner that the images5 are superimposed on one another on the entire region of theillumination zone LZ by diffraction caused by interference fringesrecorded in the hologram recording medium 55. Namely, the coherent lightbeams incident on specific positions of the hologram recording medium 55from the irradiation unit 60 are diffused, i.e. spread, by the opticaldevice 50 to be incident on the entire region in the corresponding areaof the illumination zone LZ. For example, a coherent light beam incidenton any position in the recording area r1 reproduces line images LZ1 in amanner that the line images LZ1 are superimposed on one another on theentire region of in the corresponding area in the illumination zone LZ.

In this way, the irradiation unit 60 illuminates the illumination zoneLZ with coherent light beams. For example, when the laser source 61 hasa plurality of laser sources 61 that emit light in different colors, animage 5 of the scattering plate 6 is reproduced in each color on theillumination zone LZ. Therefore, when the laser sources 61 emit lightsimultaneously, the illumination zone LZ is illuminated with white thatis a combination of three colors.

The illumination zone LZ is provided, for example, near the first endface 31 a of the light take-out portion 31. Since, the first end face 31a is provided closest to the optical device 50 in the light take-outportion 31, an illumination light beam of the illumination zone LZpropagates towards the second end face 31 b while being reflected at thethird and fourth end faces 31 c and 31 d of the light take-out portion31. One of the third and fourth end faces 31 c and 31 d, for example,the third end face 31 c, is a light take-out surface on which anillumination light beam of the illumination zone LZ is reflected andthrough which part of the illumination light beam is taken out to theoutside. With this configuration, a uniform illumination light beam canbe taken out from the entire third end face 31 c.

The illumination zone LZ may not necessarily be provided near the firstend face 31 a closest to the optical device 50 but may be providedinside the light take-out portion 31 or near the second end face 31 bfarthest from the optical device 50. For example, when the illuminationzone LZ is provided near the second end face 31 b, a coherent light beamincident on any position in each of the recording areas r1 to rn of thehologram recording medium 55 propagates into the light take-out portion31 from the first end face 31 a as diffused light and reproduces imagesof a scattering plate superimposed on one another on the entire regionin the corresponding area of the illumination zone LZ while beingreflected at the third and fourth end faces 31 c and 31 d or directlypropagating without reflection.

As described above, in the present embodiment, a plurality of recordingareas r1 to rn are formed on the hologram recording medium 55, aninterference fringe is formed on each recording area, and a plurality ofline images LZ1 to LZn are created on the illumination zone LZ. This isbecause it is presumed that the width of the first and second end faces31 a and 31 b of the light take-out portion 31 is large, for example,several ten centimeters or more. If the width of the first and secondend faces 31 a and 31 b is large, the width of the illumination zone LZis also large. However, since the diffusion angle obtained at thehologram recording medium 55 is not so large, the entire region of theillumination zone LZ may not be illuminated if there is only onerecording area. Therefore, in the present embodiment, the hologramrecording medium 55 is provided with a plurality of recording areas r1to rn. However, the hologram recording medium 55 may not be providedwith a plurality of recording areas r1 to rn if the width of the firstand second end faces 31 a and 31 b of the light take-out portion 31 issufficiently small so that the entire region of the illumination zone LZcan be covered with one recording area. In this case, coherent lightbeams emitted to any point on the hologram recording medium 55 reproduceline images in a manner that line images are superimposed on one anotheron the entire region of the illumination zone LZ.

In the present embodiment, an optical image can be created on theillumination zone LZ with speckles inconspicuous, as explained below.

According to Speckle Phenomena in Optics, Joseph W. Goodman, Roberts &Co., 2006 cited above, it is effective to integrate parameters such aspolarization, phase, angle and time to increase modes. The modes hereare speckle patterns with no correlation one anther. For example, whencoherent light beams are projected onto the same screen in differentdirections from a plurality of laser sources 61, modes exist in the samenumber as the laser sources. Moreover, when coherent light beams areprojected onto a screen in different directions per unit of time fromthe same laser source 61, modes exist by the number of changes in theincidence direction of the coherent light beams during the time that isnot covered by the resolution of human eyes. It is assumed that, ifthere are many of this mode, the interference patterns of light aresuperimposed on one another and averaged with no correlation, and as aresult, speckles observed by eyes of an observer are inconspicuous.

In the irradiation unit 60 described above, coherent light beams areemitted to the optical device 50 to scan the hologram recording medium55. Although coherent light beams incident on any positions of therecording areas r1 to rn in the hologram recording medium 55 illuminatethe entire region of the corresponding areas of the same illuminationzone LZ, the illuminating directions of the coherent light beams toilluminate the illumination zone LZ are different from one another. And,since the position on the hologram recording medium 55 on which acoherent light beam is incident changes with time, the incidencedirection of the coherent light beam on the illumination zone LZ alsochanges with time.

As described above, in the present embodiment, a coherent light beamcontinuously scans the hologram recording medium 55. Following to this,the incidence direction of a coherent light beam to the illuminationzone LZ from the irradiation unit 60 also continuously changes. When theincidence direction of a coherent light beam to the illumination zone LZfrom the optical device 50 changes slightly, for example, an angle lessthan 1°, a speckle pattern generated on the illumination zone LZ changesgreatly, resulting in superimposition of speckle patterns with nocorrelation. In addition, the frequency of a scanning device 65 such asa MEMS mirror and a polygonal mirror actually on the market is usuallyseveral hundred Hz or higher and a scanning device 65 of frequencyreaching several ten thousands Hz is not rare.

Accordingly, according to the present embodiment, the incidencedirection of a coherent light beam changes with time on each position ofthe illumination zone LZ and this change occurs at a speed that is notcovered by the resolution of human eyes. Therefore, if a screen isplaced in the illumination zone LZ, speckle patterns generatedcorresponding to respective scattering patterns are superimposed on oneanother and averaged to be observed by an observer. Accordingly,speckles become inconspicuous effectively to an observer who observes animage displayed on the screen.

According to the reason above, in the present embodiment, theillumination zone LZ is provided near the light take-out portion 31.Therefore, speckles also become inconspicuous for illumination lightbeams taken out from the light take-out portion 31.

Conventionally, speckles observed by humans are not only speckles at thelight take-out portion 31 side caused by scattering of coherent lightbeams at the light take-out portion 31 but also speckles at the opticaldevice 50 side caused by scattering of coherent light beams beforeincident on the light take-out portion 31. The speckle pattern generatedat the optical device 50 side is also recognizable to an observer bybeing taken out from the light take-out portion 31 to the outside.However, according to the embodiment described above, coherent lightbeams continuously scan the hologram recording medium 55 and each of thecoherent light beams incident on any position of each of the recordingareas r1 to rn in the hologram recording medium 55 illuminates theentire region of the corresponding area of the illumination zone LZ.Namely, the hologram recording medium 55 creates new wavefrontsdifferent from the prior wavefronts that have formed speckle patterns,that are taken out to the outside in a complex manner and uniformly viathe illumination zone LZ and the light take-out portion 31. By thecreation of new wavefronts at the hologram recording medium 55, specklepatterns generated at the optical device 50 side become invisible.

As described above, in the present embodiment, the scanning device 65makes coherent light beams scan the hologram recording medium 55 andcoherent light beams emitted from the recording areas r1 to rn in thehologram recording medium 55 are incident on the entire region in thecorresponding areas of the illumination zone LZ. With this extremelysimple configuration, uniform illumination light beams can be taken outfrom the entire region of the third end face 31 c or the fourth end face31 d of the light take-out portion 31 without speckles conspicuous.

Moreover, in the present embodiment, the laser source 61, the scanningdevice 65, the Fresnel lens 68, and the mirror member 69 are arranged atthe rear side of the surface on the opposite side to the irradiationsurface of the light take-out portion 31. With this arrangement,coherent light beams can be incident on the hologram recording medium 55from behind. Thus, the recording surface of the hologram recordingmedium can be provided roughly parallel to the first end face. Thismeans that the hologram recording medium can be provided near the lighttake-out portion 31. Therefore, the area of frame portion that does notcontribute to the illumination of the plane illumination apparatus ofFIGS. 1(a) and 1(b) can be reduced. That is, according to the presentembodiment, a plane illumination apparatus having a narrow frame can berealized. The laser source 61, the scanning device 65, the Fresnel lens68, and the mirror member 69 have to be arranged at the rear side of thesurface on the opposite side to the irradiation surface of the planeillumination apparatus. Therefore, the plane illumination apparatusbecomes longer in the depth direction by the length caused by thearrangement. However, if the length in the depth direction is not viewedas a problem, the present embodiment is effective.

Other Feature of Present Embodiment

In Speckle Phenomena in Optics, Joseph W. Goodman, Roberts & Co., 2006mentioned above, a method using a numerical value corresponding to aspeckle contrast as a parameter to indicate the degree of specklesgenerated on a screen is proposed. The speckle contrast is the quantitydefined as a value obtained by dividing the standard deviation ofvariation in intensity actually occurred on a screen by an average valueof the intensity when a test-pattern image to originally show uniformintensity distribution is displayed. A larger value of the specklecontrast means a larger degree of generation of speckles on a screen andindicates to an observer that a spotted luminance-unevenness pattern ismore remarkable.

FIG. 7 is a view showing results of measuring speckle contrasts in thecases where the hologram recording medium 55 described above was usedand not used. FIG. 7(a) shows a result obtained when a laser beam wasdirectly emitted to the illumination zone LZ without using the scanningdevice 65 and the optical device 50. FIG. 7(b) shows a result obtainedwhen a volume hologram produced having a diffusion angle of 20° was usedas the optical device 50. FIG. 7(c) shows a result obtained when arelief diffusion plate was used as the optical device 50. FIG. 7(d)shows a result obtained when a singe-color LED light beam was directlyemitted to the illumination zone LZ by using a singe-color LED, insteadof the laser source 61, as the irradiation unit 60.

When it is considered that a speckle contrast of 5 or lower is areference index indicating an allowable speckle noise in a displayapparatus or the like, in the present embodiment of the presentinvention shown in FIG. 7(b), a speckle contrast was lower than 4, andhence extremely favorable result was obtained.

The problem of generation of speckles is practically a problem unique tothe case of using a coherent light source of a laser beam or the like,and thus, the problem needs not be considered in the case of anapparatus using an incoherent light source such as an LED. However,according to FIG. 7, the present embodiment is superior to the caseusing a singe-color LED concerning the speckle contrast. The reason maybe that a light diffusion device 21 is not used for illumination for asinge-color LED. As described above, It can be said that the planeillumination apparatus according to the present embodiment sufficientlydealt with the speckle defect.

In addition, according to the present embodiment described above, thefollowing advantages can be obtained.

According to the present embodiment described above, the optical device50 for making speckles inconspicuous can also function as an opticalmember for shaping and adjusting the beam shape of a coherent light beamemitted from the irradiation unit 60. Therefore, it is possible tominiaturize and simplify the optical module.

Moreover, according to the present embodiment described above, coherentlight beams incident on specific positions in the recording areas r1 torn of the hologram recording medium 55 create images of the scatteringplate 6 in respective colors on the entire region of the correspondingareas of the illumination zone LZ. Therefore, it is possible to utilizeall of the light beams diffracted by the hologram recording medium 55for illumination, thus excellent in utilization efficiency of lightbeams from the laser source 61.

[Avoidance of Zero-Order Light]

Part of coherent light beams from the irradiation unit 60 is notdiffracted by the hologram recording medium 55 but passes though it.This type of light is called zero-order light. When zero-order light isincident on the illumination zone LZ, an abnormal region, i.e. a spottedregion, a line region, and a plane region, inevitably appears in whichbrightness, i.e. intensity, is rapidly increased compared with thesurroundings.

When the reflection-type hologram recording medium 55, hereinafter,“reflection-type holograms”, is used, the illumination zone LZ and thelight take-out portion 31 are not arranged in a propagation direction ofzero-order light, hence it is relatively easy to avoid zero-order light.However, when the transmission-type hologram recording medium 55,hereinafter, “transmission-type holograms” is used, in the presentembodiment, it is difficult to have a configuration for avoidingzero-order light because the distance from the transmission-typehologram to the first end face is short and the recording areas r1 to rnof the transmission-type hologram are aligned in one axis direction.Therefore, in the case of the transmission-type holograms, it isdesirable to raise diffraction efficiency as high as possible torestrict the effects of zero-order light as much as possible.

[Reflection- and Transmission-Type Hologram Recording Media 55]

Reflection-type holograms show higher wavelength selectivity thantransmission-type holograms. In other words, in reflection-typeholograms, although interference fringes corresponding to differentwavelengths are superimposed on one another in layers, a coherent lightbeam having a desired wavelength can be diffracted by a desired layeronly. In addition, reflection-type holograms are excellent in that theinfluence of zero-order light can be easily removed.

On the other hand, although transmission-type holograms have a widespectrum range for diffraction and a high acceptable level to the lasersource 61, if interference fringes corresponding to differentwavelengths are superimposed on one another in layers, layers other thana desired layer also diffract coherent light of a desired wavelength.Therefore, in general, it is difficult to configure transmission-typeholograms in a layered structure.

In the plane illumination apparatus of FIGS. 1 and 2, thereflection-type hologram recording medium 55 is provided oblique to thefirst end face 31 a of the light take-out portion 31. When atransmission-type hologram 55 is used, this hologram 55 may be providedalmost parallel with the first end face 31 a and a reflection member(not shown) for reflecting again coherent light beams reflected by thescanning device 65 to guide the coherent light beams to the hologramrecording medium 55, that is, a propagation-direction changing memberfor changing again the propagation directions of coherent light beamshaving the propagation directions changed once by the scanning device 65may be newly provided.

(Irradiation Unit 60)

The embodiment described above shows an example in which the irradiationunit 60 includes the laser source 61 and the scanning device 65. In theexample, the scanning device 65 includes the one-axis-rotation typemirror device 66 that changes the propagation direction of a coherentlight beam by reflection. However, the scanning device 65 is not limitedthereto. As shown in FIG. 8, the scanning device 65 may be configured sothat the mirror, i.e. reflection plane 66 a, of the mirror device 66 canrotate about the first rotation axis line RA1 as well as about a secondrotation axis line RA2 intersecting the first rotation axis line RA1. Inthe example shown in FIG. 8, the second rotation axis line RA2 of themirror 66 a is perpendicular to the first rotation axis line RA1 whichis extended in parallel to the Y axis of the XY coordinate systemdefined on the plate plane of the hologram recording medium 55. Then,since the mirror 66 a can rotate about both of the first axis line RA1and the second axis line RA2, the incidence point IP of a coherent lightbeam from the irradiation unit 60 incident on the optical device 50 canbe shifted on the plate plane of the hologram recording medium 55 intwo-dimensional directions. Therefore, as an example, as shown in FIG.8, the incidence point IP of a coherent light beam incident on theoptical device 50 can be shifted along a circumference.

Moreover, the scanning device 65 may include two or more mirror devices66. In this case, although the mirror 66 a of the mirror device 66 canrotate about only a single axis line, the incidence point IP of acoherent light beam from the irradiation unit 60 incident on the opticaldevice 50 can be shifted on the plate plane of the hologram recordingmedium 55 in two-dimensional directions.

As a concrete example of the mirror device 66 a included in the scanningdevice 65, there are a MEMS mirror, a polygonal mirror, and the like.

Moreover, the scanning device 65 may be configured to include otherdevices other than a reflection device, for example, the mirror device66 described above, which changes the propagation direction of acoherent light beam by reflection. For example, the scanning device 65may include a refraction prism, a lens, etc.

Essentially, the scanning device 65 is not a necessary component. Thelight source 61 of the irradiation unit 60 may be configured so thatthey can be displaced, i.e. moved, oscillated, and rotated, with respectto the optical device 50. Coherent light beams emitted from the lightsource 61 may scan the hologram recording medium 55 in accordance withthe displacement of the light sources 61 with respect to the opticaldevice.

Moreover, although the description hereinbefore is made on conditionthat the light source 61 of the irradiation unit 60 oscillates a laserbeam shaped into a line beam, the preset invention is not limitedthereto. Particularly, in the embodiments described above, coherentlight beams emitted to respective positions of the optical device 50 areshaped by the optical device 50 into a light flux which is incident onthe entire region of the illumination region LZ. Therefore, no problemoccurs even if coherent light beams emitted from the light source 61 ofthe irradiation unit 60 to the optical device 50 are not accuratelyshaped. For this reason, coherent light beams generated from the lightsource 61 may be diverging light. In addition, the shape of coherentlight beams, in cross section, generated from the light sources 61 maybe an ellipse or the like instead of a circle. In addition, thetransverse mode of coherent light beams generated from the light source61 may be a multi-mode.

In addition, when the light source 61 generates a diverging light flux,coherent light beams are incident on the hologram recording medium 55 ofthe optical device 50 not on a spot but on a region having a certainarea. In this case, light beams which are diffracted by the hologramrecording medium 55 and incident on respective positions of theillumination region LZ are angularly-multiplexed. In other words, ineach instant, on respective positions of the illumination region LZ,coherent light beams are incident from directions within a certain anglerange. Due to the angle-multiplexing, it is possible to more effectivelymake speckles inconspicuous.

Moreover, in the embodiments described above, although the example isdescribed in which the irradiation unit 60 emits a coherent light beamto the optical device 50 so that the coherent light beam traces theoptical path of one beam included in a light flux, the present inventionis not limited thereto. For example, in the above embodiments, as shownin FIG. 9, the scanning device 65 may further include a condenser lens67 disposed at the downstream side of the mirror device 66 along theoptical path of a coherent light beam. In this case, a light beam fromthe mirror device 66, which propagates along the optical path of lightbeams that form a light flux, becomes a light beam that propagates in acertain direction through the condenser lens 67. In other words, theirradiation unit 60 emits a coherent light beam to the optical device 50so that the coherent light beam traces the optical path of one beamincluded in a light flux. In this kind of example, instead of aconverging light flux described above, a parallel light flux is used asthe reference light beam Lr in the exposure process in the production ofthe hologram recording medium 55. The hologram recording medium 55described above can be more simply produced and replicated.

(Optical Device 50)

In the embodiments described above, although the example in which theoptical device 50 is configured with a reflection-type volume hologramrecording medium 55 using photopolymer has been described, the presentinvention is not limited thereto. Moreover, the optical device 50 mayinclude a volume hologram recording medium that is a type in whichrecording is performed by using a photosensitive medium including asilver halide material. Moreover, the optical device 50 may include atransmission-type volume hologram recording medium 55 or a relief-type,i.e. emboss-type hologram recording medium 55.

With respect to the relief-type, i.e. emboss-type, hologram recordingmedium, a hologram interference fringe is recorded using aconvex-concave structure of the surface thereof. However, in the case ofthe relief-type hologram recording medium, scattering due to theconvex-concave structure of the surface may also cause generation of newspeckles, hence in this respect, the volume hologram recording medium ispreferable. In the case of the volume hologram recording medium, ahologram interference fringe is recorded as a refractive indexmodulation pattern, i.e. refractive index distribution, of an innerportion of the medium, hence there is no influence of scattering becauseof the convex-concave structure of the surface.

However, even when the volume hologram recording medium is used, a typein which recording is performed using a photosensitive medium includinga silver halide material may become a cause of generating new specklesdue to scattering of silver halide particles. In this respect, thevolume hologram recording medium using a photopolymer is preferable asthe hologram recording medium 55.

Moreover, in the recording process shown in FIG. 4, although a so-calledFresnel-type hologram recording medium 55 is produced, a Fouriertransform-type hologram recording medium 55 which can be obtainedthrough recording using lenses may be produced. When the Fouriertransform-type hologram recording medium is used, lenses can also beused for image reproduction.

In addition, a striped pattern, i.e. refractive index modulation patternor convex-concave pattern, which is to be formed on the hologramrecording medium 55 may be designed by using a computer based on aplanned wavelength or incidence direction of a reproduction illuminationlight beam La, a shape or position of an image to be reproduced, and thelike, without use of an actual object light beam Lo and reference lightbeam Lr. The hologram recording medium 55 obtained in this manner iscalled a computer generated hologram recording medium. Moreover, when aplurality of coherent light beams having mutually different wavelengthranges are emitted from the irradiation unit 60 in a similar manner inthe modification described above, the hologram recording medium 55 as acomputer generated hologram recording medium may be partitionedtwo-dimensionally into a plurality of regions provided corresponding tocoherent light beams of respective wavelength ranges so that thecoherent light beams of the respective wavelength ranges are diffractedin the corresponding regions to reproduce images.

Moreover, in the embodiments described above, although the example isdescribed in which the optical device 50 includes the hologram recordingmedium 55 by which coherent light beams emitted to respective positionsthereof are spread to illuminate the entire region of the illuminationregion LZ, the present invention is not limited thereto. Instead of thehologram recording medium 55 or in addition to the hologram recordingmedium 55, the optical device 50 may include a lens array as an opticaldevice by which the propagation directions of coherent light beamsincident on respective positions thereof are changed and the coherentlight beams are diffused to illuminate the entire region of theillumination region LZ. As a concrete example of the lens array, a totalreflection-type or refraction-type Fresnel screen having a diffusingfunction, a fly-eye lens, and the like may be exemplified. In this typeof illumination apparatus 40, the irradiation unit 60 and the opticaldevice 50 may be configured so that the irradiation unit 60 emitscoherent light beams to the optical device 50 so that the coherent lightbeams scan the lens array and the propagation directions of the coherentlight beams incident on respective positions of the optical device 50from the irradiation unit 60 are changed by the lens array, then thecoherent light beams having the propagation directions changedilluminate the illumination region LZ, thus effectively making specklesinconspicuous.

(Illuminating Method)

In the embodiments described above, an example is shown in which theirradiation unit 60 is configured to be able to scan the optical device50 in a one-dimensional direction with coherent light beams and thehologram recording medium 55 or the lens array of the optical device 50is configured to diffuse, i.e. spread and diverge the coherent lightbeams incident on respective positions of the hologram recording medium55 in a two-dimensional direction, so that the illumination apparatus 40illuminates the two-dimensional illumination region LZ. However, asdescribed above, the present invention is not limited to such example.For example, the irradiation unit 60 may be configured to be able toscan the optical device 50 in a two dimensional direction with coherentlight beams and the hologram recording medium 55 or the lens array ofthe optical device 50 may be configured to diffuse, i.e. spread anddiverge, the coherent light beams incident on respective positions ofthe hologram recording medium 55 in a two-dimensional direction, so thatthe illumination apparatus 40 illuminates the two-dimensionalillumination region LZ, as shown in FIG. 8.

Moreover, as already described, the irradiation unit 60 may beconfigured to be able to scan the optical device 50 in a one-dimensionaldirection with coherent light beams and the hologram recording medium 55or the lens array of the optical device 50 may be configured to diffuse,i.e. spread and diverge, the coherent light beams incident on respectivepositions of the hologram recording medium 55 in a one-dimensionaldirection, so that the illumination apparatus 40 illuminates theone-dimensional illumination region LZ. In this configuration, thescanning direction of a coherent light beam from the irradiation unit 60and the diffusing direction, i.e. spreading direction, by the hologramrecording medium 55 or the lens array of the optical device may beparallel with each other.

Furthermore, the irradiation unit 60 may be configured to be able toscan the optical device 50 in a one- or two-dimensional direction withcoherent light beams and the hologram recording medium 55 or the lensarray of the optical device 50 may be configured to diffuse, i.e. spreadand diverge, the coherent light beams incident on respective positionsof the hologram recording medium 55 in a one-dimensional direction. Inthis configuration, as already described, the optical device 50 may havea plurality of hologram recording media 55 or lens arrays to illuminateillumination zones LZ corresponding to the hologram recording media 55or lens arrays successively, so that the illumination apparatus 40illuminates a two-dimensional region. In this occasion, the illuminationzones LZ may be successively illuminated at a speed felt like as ifsimultaneously illuminated for human eyes or at a low speed so thathuman eyes can recognize that the illumination zones LZ are successivelyilluminated. In other words, the recording areas r1 to rn describedabove may be formed by using one hologram recording medium 55 or aplurality of recording media may be produced by using different hologramrecording media 55, respectively.

The present invention is not limited to the embodiments described abovebut includes various modifications conceivable by those skilled in theart. The effects of the present invention are also not limited to thosedescribed above. Namely, various additions, modifications and partialomissions may be made without departing from the conceptual idea andgist of present invention derived from those defined in the accompanyingclaims and their equivalents.

The invention claimed is:
 1. A plane illumination apparatus comprising:an optical device configured to be capable of diffusing coherent lightbeams from respective points to an entire region of the correspondingareas in an illumination zone and to change an incident direction of thecoherent light beams incident on respective points of the illuminationzone with time so that speckle patterns on the illumination zone aresuperimposed on one another and averaged; an irradiation unit configuredto irradiate the coherent light beams to the optical device so that thecoherent light beams scan a surface of the optical device; and a lightguide plate configured to make coherent light beams that are reflectedat a surface of the optical device or that have passed through theoptical device propagate and to comprise a light take-out surface fromwhich the coherent light beams are taken outside; wherein theirradiation unit makes the coherent light beams scan the surface of theoptical device by changing propagation directions of the coherent lightbeams; wherein the light guide plate comprises a light take-out portionconfigured to take coherent light beams outside while making coherentlight beams propagate between a first end face on which coherent lightbeams from the optical device are incident and a second end face that isprovided to face the first end face; wherein the illumination zone isprovided inside the light take-out portion or along the first end face,or along the second end face; wherein the light take-out surface is athird end face that is connected to the first and second end faces;wherein the irradiation unit is provided at a rear side of a fourth endface that is an opposite side with respect to the light take-out surfaceof the light take-out portion; and wherein the irradiation unit furthercomprises a light source to emit coherent light beams, and a scanningdevice to make the coherent light beams emitted from light sourceperform scanning on a surface of the optical device by changingpropagation directions of the coherent light beam.
 2. The planeillumination apparatus of claim 1, wherein the optical device isprovided in contact with one end face of the light guide plate, andcoherent light beams diffused by the optical device are incident on theone end face of the light guide plate and reflected totally at twoopposing surfaces of the light guide plate, or the coherent light beamsare directly incident on the first end face of the light guide plate. 3.The plane illumination apparatus of claim 1, wherein the optical deviceis provided apart from the light guide plate, and wherein at least partof coherent light beams diffused by the optical device is incident onthe first end face of the light guide plate.
 4. The plane illuminationapparatus of claim 1 comprising a divergence-angle restricting partconfigured to restrict divergence angles of coherent light beams havingpropagation directions changed by the irradiation unit, wherein coherentlight beams having divergence angles restricted by the divergence-anglerestricting part are incident on the optical device.
 5. The planeillumination apparatus of claim 4, wherein the divergence-anglerestricting part is a Fresnel lens provided at a rear side of the fourthend face.
 6. The plane illumination apparatus of claim 1 comprising areflection member configured to reflect coherent light beams havingpropagation directions changed by the irradiation unit to make thecoherent light beams incident on a surface of the optical device.
 7. Theplane illumination apparatus of claim 1, wherein the optical device isprovided roughly parallel with the first end face.
 8. The planeillumination apparatus of claim 1, wherein the optical device comprisesa plurality of recording areas that are scanned by coherent light beamshaving propagation directions changed within different angle ranges bythe irradiation unit, that are incident on the plurality of recordingareas at respective different angles or at a same angle, wherein theillumination zone comprises a plurality of image reproduction areascorresponding to the plurality of recording areas, respectively, whereinthe plurality of recording areas correspond to the plurality of imagereproduction areas, respectively, and wherein an interference fringe isrecorded in each of the plurality of recording areas, to diffusecoherent light beams on the entire region in the corresponding imagereproduction area.
 9. The plane illumination apparatus of claim 8,wherein the plurality of illumination zones are arranged next to oneanother or arranged to be partially overlapped with one another so as tobe irradiated with coherent light beams of uniform brightness from thelight take-out surface.
 10. The plane illumination apparatus of claim 8,wherein the plurality of recording areas are configured by using onehologram recording medium.
 11. The plane illumination apparatus of claim8, wherein the plurality of recording areas are configured by usingdifferent hologram recording media, respectively.
 12. The planeillumination apparatus of claim 1, wherein the light guide platecomprises a fifth and a sixth end face that are connected to the firstto fourth end faces, at least one of the second, fifth and sixth endfaces having a mirror surface for reflecting coherent light beamsincident and propagating from the first end face.
 13. The planeillumination apparatus of claim 1, wherein the optical device is areflection-type hologram recording medium provided roughly parallel withthe first end face.
 14. The plane illumination apparatus of claim 1,wherein the optical device is a transmission-type hologram recordingmedium and the irradiation unit comprises: a light source configured toemit coherent light beams; a scanning device configured to make thecoherent light beams emitted from light source perform scanning on asurface of the optical device by changing propagation directions of thecoherent light beam; and a propagation-direction changing memberconfigured to change again the propagation directions of the coherentlight beam changed by the scanning device, to guide the changed againcoherent light beam to the hologram recording medium.
 15. A backlightapparatus provided with a plane illumination apparatus, the planeillumination apparatus comprising: an optical device configured to becapable of diffusing coherent light beams from respective points to anentire region of the corresponding areas in an illumination zone and tochange an incident direction of the coherent light beams incident onrespective points of the illumination zone with time so that specklepatterns on the illumination zone are superimposed on one another andaveraged; an irradiation unit configured to irradiate the coherent lightbeams to the optical device so that the coherent light beams scan asurface of the optical device; and a light guide plate configured tomake coherent light beams that are reflected at the surface of theoptical device or that have passed through the optical device propagateand to comprise a light take-out surface from which the coherent lightbeams are taken outside; wherein the irradiation unit makes the coherentlight beams scan the surface of the optical device by changingpropagation directions of the coherent light beams; wherein the lightguide plate comprises a light take-out portion configured to takecoherent light beams outside while making coherent light beams propagatebetween a first end face on which coherent light beams from the opticaldevice are incident and a second end face that is provided to face thefirst end face; wherein the illumination zone is provided inside thelight take-out portion or along the first end face, or along the secondend face; wherein the light take-out surface is a third end fact that isconnected to the first and second end faces; wherein the irradiationunit is provided at a rear side of a fourth end face that is an oppositeside with respect to the light take-out surface of the light take-outportion; and wherein the irradiation unit comprises a light source toemit coherent light beams, and a scanning device to make the coherentlight beams emitted from light source perform scanning on a surface ofthe optical device by changing propagation directions of the coherentlight beam.